The blur itself was already parallel (9-15 ms on 24 MP); most of the
command was serial glue around it. Modify copied the mask into a padded
buffer and back out sample by sample even with no padding, and combine
built a full-canvas byte buffer, scanned it, then copied it into tiles
on one thread.
- modify: use the mask as is when the selection is clear of the canvas
edge (pad 0); pad and crop row by row otherwise.
- combine / mask_to_surface: fill selection tiles in parallel straight
from the mask, a row at a time; fully selected tiles share one
copy-on-write tile.
Bit-identical: the per-sample oracles (combine, now with whole tiles,
and the old pad/crop) are compared in tests.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
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